1. Field of the Invention
This invention relates to a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), and more particularly to a MOSFET having a very small amount of minute current.
The minute current which is so called a tailing current is a drain current at the vicinity of a threshold voltage of the MOSFET, that is, from about 10.sup.-.sup.6 A to about 10.sup.-.sup.12 A.
It is generally said that, in a MOSFET, the drain current is proportional to the square of the gate voltage, but practically a minute drain current flows even when the gate voltage is lower than the threshold voltage. This minute drain current is a so-called tailing current and decreases according to the gate voltage. In the MOSFET, however, there is a phenomenon that the drain current is fixed at a certain very minute value which is called hereinafter as a residual current.
These tailing and residual currents become one of causes for errors in circuits and are fatal to devices utilizing MOSFETs. For example, in a dynamic type memory device utilizing MOSFETs, since the tailing current and the residual current easily flow between the source and drain regions of the MOSFETs even when the value of the gate voltages of the MOSFETs is lower than the threshold voltage of the MOSFETs which is usually defined by a gate voltage at a drain current of about 10.sup.-.sup.6 A, and hence charges for establishing memory functions in the memory device flow out, the memory device cannot sufficiently function. Further, these MOSFETs consume a large amount of electricity, since minute current always flows during the operation thereof.
Further, in an integrated circuit device, it has been desired to change an enhancement type MOSFET to a depletion type MOSFET or a depletion type MOSFET to an enhancement type MOSFET for reducing the steps for fabricating the integrated circuit device and a MOSFET has been proposed, wherein a semiconductor region having the same or opposite conductivity type to that of the semiconductor body of MOSFET is disposed in the body between source and drain regions. Although the threshold voltage of the MOSFET is easily changed from an inherent value of the MOSFET which is determined by the impurity concentration of the body to a desired value by disposing the semiconductor region having a desired impurity concentration, there is such a defect that the larger the difference between the inherent value and the desired value of the threshold voltage is, the larger the tailing current and the residual current become.